1. Field of the Invention
The present invention relates to a nickel-metal hydride storage battery and particularly to an improvement in its negative electrode essentially consisting of a hydrogen storage alloy.
2. Background Art
The nickel-metal hydride storage battery that uses a negative electrode including a hydrogen storage alloy is widely used as a power source for various kinds of wireless devices or electronic devices, such as telecommunication equipment and personal computers, because it is more eco-friendly and has higher energy density than the conventional nickel-cadmium storage battery. Further, the nickel-metal hydride storage battery is also used in an electric power tool or electric car for which high-current charge/discharge is essential. Because applications of the nickel-metal hydride storage battery are expanding as described above, further improvements in its charge/discharge characteristics are desired.
A description is provided of a process of manufacturing a conventional negative electrode including a hydrogen storage alloy. First, a binder, e.g. polytetrafluoro-ethylene, and styrene-butadiene rubber, and a viscosity improver, e.g. methyl cellulose, carboxyl methyl cellulose, and polyvinyl alcohol, are added to a pulverized hydrogen storage alloy powder, to prepare a mixture. Next, the mixture is kneaded with water, to provide a paste. Thereafter, the paste is applied to a punching metal, i.e. a core material, dried and pressed.
For a negative electrode made of a paste type hydrogen storage alloy manufactured as above, coating the surface of the hydrogen storage alloy powder in the negative electrode with a binder is likely to decrease the conductivity between the hydrogen storage alloy particles. When the conductivity decreases, an increase in the rate of hydrogen storage alloy particles uninvolved in charge/discharge increases the inner pressure of the battery during overcharge, and decreases the high-current discharge characteristics. This poses a problem of decreasing the cycle life of the battery.
To address the above problem, the following prior arts are disclosed. In the method described in Japanese Patent Unexamined Publication No. H11-185745, carbon powder is added to the negative electrode as a conductive agent. The addition of carbon powder increases the conductivity between hydrogen storage alloy particles, and the effect of excellent gas compatibility carbon material inherently has promotes gas consumption reaction of the negative electrode. These effects can alleviate an increase in the inner pressure of the battery and increase the high-current discharge characteristics to some degrees. However, because the conductivity of carbon powder is insufficient the effects of inhibiting an increase in the inner pressure, and an improvement in the high-current discharge characteristics are not sufficient.
In the method described in Japanese Patent Unexamined Publication No. H11-111298, metal coated carbon powder in which a metal is coated on at least a part of the surface of a carbon particle is added. High conductivity of the metal coating the surface of carbon particles can increase the conductivity of the negative electrode, improve the high-current discharge characteristics, and inhibit an increase in the inner pressure of the battery. However, the metal coating inhibits the excellent gas compatibility carbon material inherently has, and thus the effect of inhibiting an increase in the inner pressure of a battery is small. This phenomenon is conspicuous particularly when the coating area is large. In contrast, when the coating area is too small, the difference from ordinary carbon powder is small, and the effect of improving the battery characteristics is difficult to obtain.
Japanese Patent Unexamined Publication No. H07-65826 discloses a method of adding nickel powder containing 0.2 to 3 wt % of carbon. This method prevents oxygen from oxidizing the negative electrode, and improves the conductivity between the hydrogen storage alloy particles. This can inhibit an increase in the inner pressure of the battery, and improve the high-current discharge characteristics. However, because the main body of the additive is a metal powder, the effect of the excellent gas compatibility carbon material inherently has is not fully exercised, and thus inhibition of an increase in the inner pressure of the battery must be improved.
In the above prior arts, although the conductivity of the negative electrode including a hydrogen storage alloy may be improved, the degree of improving the conductivity and electrode reaction are insufficient. Therefore, the effects of inhibiting an increase in the inner pressure of the battery and improving the high-current discharge characteristics are also insufficient.